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Procedia Engineering 72 ( 2014 ) 80 – 85

The 2014 conference of the International Sports Engineering Association Development of a dynamometer to measure grip forces at a handlebar

Dominik Krumma,*, Stephan Odenwalda

aTechnische Universität Chemnitz, Department of Sports Equipment and Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany

Abstract

The purpose of this paper was to develop a dynamometer to record grip forces at the human-machine interface in biomechanical studies. The development of the dynamometer was based on the approach described in VDI 2221. The principal solution was a dynamometer in the shape of a hollow cylinder. The dynamometer contains one clamp and two facing bending beams in the shape of half-shells which were equipped with strain gages allowing measurements of deformations. The validation was carried out in a biomechanical study, indicating a maximum relative error of 4.5 %. The reproducibility of measurements and the functionality was proven.

© 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Centre for Sports Engineering Research, Sheffield Hallam University. Selection and peer-review under responsibility of the Centre for Sports Engineering Research, Sheffield Hallam University

Keywords: dynamometer; human-machine interface; grip forces; strain gages; biomechanical study

1. Introduction

Epidemiological studies have shown that prolonged, occupational exposure to hand-transmitted vibration is associated with the hand-arm vibration syndrome (Bernard, 1997). The main sources of vibration exposure are vibrating handheld tools. The vibration transmission from a tool handle to the hand-arm system depends on the mechanical impedance of the system (Dong et al., 2005). The mechanical impedance is defined as the complex ratio of transmitted force and vibration velocity at the driving point (ISO 10068). Grip strengths of workers as well

* Corresponding author. Tel.: +49-371-531-38528; fax: +49-371-531-23149. E-mail address: [email protected]

1877-7058 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Centre for Sports Engineering Research, Sheffield Hallam University doi: 10.1016/j.proeng.2014.06.017 Yvan Champoux and Stephan Odenwald / Procedia Engineering 72 ( 2014) 80 – 85 81 as of patients with musculoskeletal disorders have been comprehensively investigated by ergonomics and physicians (Dong et al., 2008; Radhakrishnan and Nagaravindra, 1993). However, the mechanical impedances of athletes during sporting activities have been hardly investigated. The original intention for the development of the new dynamometer was to investigate the effects of elastic compression sleeves on the biodynamic response of the hand-arm system to vibrations during . To evaluate the mechanical impedance of subjects during cycling, researchers are reliant on a dynamometer that could either be used in combination or in exchange of a bicycle handlebar to control the gripping forces of the subjects. Based on a review by Roberts et al. (2011) the most widely used dynamometer is the Jamar hand dynamometer (Lafayette Instrument Company, Lafayette, IN, US). The commercial dynamometer is based on a hydraulic system. The device provides an adjustable handle for five gripping positions and measures forces in a single axis (Edgren et al., 2004). Although, the Jamar hand dynamometer is accepted as the gold standard (Balogun et al., 1991; Roberts et al., 2011), the dynamometer has some limitations. Like most other commercial hand-held dynamometers, the device is designed to measure only maximal forces, provides only one degree of freedom, and cannot be used in combination with further items such as a handlebar or handheld tool (Chadwick and Nicol, 2001). Hence, there are also several self-made hand-held dynamometers in use that were designed to fulfil task specific requirements (Amis and Amis, 1987; Burström, 1994; Chadwick and Nicol, 2001; Dong et al., 2005; Edgren et al., 2004; Irwin and Radwin, 2007; Marcotte et al., 2005; McGorry, 2001; Radwin et al., 1991; Wimer et al., 2009). Nevertheless, literature research has shown that neither existing commercial nor existing self-made hand-held dynamometers were suitable for measuring grip forces at the interface between biker and bicycle. Hence, the purpose of this paper was to develop a robust dynamometer that records gripping forces at the human-machine interface hand-handlebar. In detail, the newly designed device should be capable of independently measuring gripping and supporting forces at a bicycle handlebar while being exposed to vibrations.

2. Methods

The development of the dynamometer was based on the approach described in VDI 2221. The approach included (i) construction, (ii) manufacture, (iii) implementation, and (iv) proof of functionality. The construction process comprised of three steps. In the first step, i.e. planning and task clarification, the requirements of the novel dynamometer were identified by literature review and discussions within our research group. During the second step, i.e. conceptual design, the principal solution for the development of the dynamometer was determined. For this purpose, a function structure representing the solution-neutral relationship between inputs and outputs was established. Suitable working principles were identified and combined into a working structure using a morphological matrix (Zwicky, 1989). Elaborated solution variants were evaluated based on the appraisal procedure of Kesselring (VDI 2225-3). Finally, the best principal solution was selected. In the last step, i.e. embodiment and final design, the principal solution was transferred into an overall layout design. After revision of the overall layout design, the final layout design was determined and technical drawings were prepared. The implementation process comprised of the calibration and validation of the new developed dynamometer with a custom test rig. For determination of the calibration curve, both half-shells of the dynamometer were independently loaded at their centre, i.e. at a distance of 60 mm to the distal end of the dynamometer, with weight plates ranging from approximately 5 kg to 45 kg. The precise weights of the weight plates were measured with a calibrated scale (PCE-PB 150, PCE Group, Meschede, Germany). The change in electrical resistance of the applied strain gages of either the top or the bottom half-shell was recorded with a measuring amplifier (Cronos 7008, imc Meßsysteme GmbH, Berlin, Germany) at a sampling rate of 100 Hz. The relation between the change in electrical resistance and the applied load was determined by means of linear regression of the recorded values. The validation contained measurements about relative error, reproducibility, crosstalk, creep, and line of force application. For the evaluation of the relative error, the measurements performed during calibration were repeated. The relative HUURU į ZDVWKHQFDOFXODWHGDV 82 Yvan Champoux and Stephan Odenwald / Procedia Engineering 72 ( 2014) 80 – 85

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ZKHUH Ȟm LV WKH PHDVXUHG IRUFH DQG Ȟa is the acting force of the calibration measurement. To evaluate the reproducibility of the dynamometer, another series of measurements were performed by a second investigator. The reproducibility was quantified as VWDQGDUGGHYLDWLRQ ı of measured forces. Since the voltage changes of the non- loaded half-shells were also consistently recorded, the crosstalk was observed from the same measurements. For creep evaluation, the voltage change of the unloaded dynamometer was recorded over a period of 30 minutes. The extent of creep was calculated as the difference between the initial value and the final value. To evaluate the influence of the line of force application, the dynamometer was loaded with a weight plate of approximately 10 kg at five different positions. The positions were at a distance of 5 mm, 30 mm, 60 mm, 90 mm, and 115 mm to the distal end of the dynamometer, respectively. To proof the functionality of the novel dynamometer a biomechanical pilot study with one volunteer cycling on a was performed under laboratory conditions. The front of the bike was fixed to an oscillating platform. The rear wheel was fixed to a cycle ergometer (T1680 Flow Ergotrainer, Tacx by, Wassenaar, The Netherlands). The handlebar was equipped with the new developed dynamometer and a laptop in front of the volunteer was used to provide feedback about the present supporting and gripping forces. In total, eight measurements with different tasks were covered by the pilot study (cf. Table 3).

3. Results

A list of requirements (Table 1) and a function structure were established. Seven potential principal solutions with scores ranging from 2.2 points up to 3.55 points were identified from a morphological matrix. The potential principal solution with the highest score was selected for embodiment and final design. The principal solution was a gripping body in the shape of a hollow cylinder made of aluminium. The dynamometer contains on the gripping side two facing bending beams in the shape of half-shells and on the non-gripping side one clamp (Fig. 1).

Table 1. Requirements list for the development of a dynamometer that measures grip forces at a bicycle handlebar.

requirements

independent monitoring of gripping and supporting forces of the right hand (D) and/or the left hand (W) compatibility with a mountain bike handlebar (D) and further bike handlebars (W) monitoring forces independently from the line of force application (D) determination of the supporting force direction (W) measuring range from 0 N to 500 N (D) robust design against vibrations (D) relative error of less than 5 % full scale (D) or less than 2 % full scale (W)

Note D indicates demands and W indicates wishes.

The bending beams were equipped with four shear strain gages (1-XY23-1.5/120, HBM Deutschland, Darmstadt, Germany). The strain gages were symmetrically applied in the neutral axis of the beams. Two gages were interconnected in a full-bridge circuit to cover the strains of the top half-shell (THS) and two gages were interconnected in a full-bridge circuit to cover the strains of the bottom half-shell (BHS). The arrangement allows measurements of supporting forces, i.e. forces that were applied radial to the THS, and of gripping forces, i.e. the sum of forces that were applied radial to the THS and BHS. Furthermore, the arrangement allows for thermal expansion compensation, compensation of forces applied at the front-side, and torsion compensation. The measuring range of the dynamometer is from 0 N to 500 N for supporting forces and from 0 N to 1000 N for Yvan Champoux and Stephan Odenwald / Procedia Engineering 72 ( 2014) 80 – 85 83 combined supporting and gripping forces. The clamp can be used to mount the dynamometer to a bike handlebar with diameters less than 23 mm.

Fig. 1. Novel dynamometer (a) mounted on a mountain bike handlebar and gripped by the right hand; (b) principal solution indicating the location of the strain gages; (c) scheme of the full bridge circuit used for the top and bottom half-shell.

The measured weights of the weight plates deviated maximal 2.7 % from the manufacture’s data (Table 2a). The calibration measurements showed a linear behaviour for both half-shells (Fig. 2). The validation indicated a maximum absolute error of 2.9 N for THS and of 3.6 N for BHS. The maximum relative error was 4.5 % for THS and for BHS (Table 2b). The reproducibility measurements resulted in a maximum standard deviation of 0.71 N (Table 2c). Crosstalk and creep were within the limit of 1 N. The measurements performed with the dynamometer were dependent from the line of force application (Fig. 3). The absolute deviation from the reference value of 99 N measured at the dynamometer’s centre was 15 N or 15.2 %. The device proved its functionality in the pilot test (Table 3). The exposure of the dynamometer to vibrations indicated no deviations compared to measurements without exposure to vibration.

Table 2. Validation of the new developed dynamometer (a) characterization of the used weight plates; (b) measurements performed by investigator A to determine the relative error and crosstalk; (c) measurements performed by investigator B to determine the reproducibility. a) weight characterization b) investigator A c) investigator B load on THS load on BHS load on THS load on BHS manufacture’s scale weight į true load THS BHS į THS BHS į THS ı BHS ı data [kg] [kg] [%] [N] [N] [N] [%] [N] [N] [%] [N] [N] [N] [N] 5 4.87 2.7 48 50 0 4.5 50 0 4.5 50 0 50 0 10 9.90 1.0 97 100 0 2.9 99 0 1.9 99 0.71 100 0.71 15 14.77 1.6 145 147 0 1.4 148 0 2.1 147 0 148 0 20 19.50 2.6 191 194 1 1.4 194 0 1.4 193 0.71 195 0.71 25 24.37 2.6 239 242 0 1.2 242 0 1.2 241 0.71 242 0 30 29.40 2.0 288 291 1 0.9 292 1 1.2 291 0 291 0.71 35 34.72 0.8 341 339 1 0.5 340 0 0.2 338 0.71 340 0 40 39.00 2.6 383 385 1 0.6 386 1 0.9 n.m. - n.m. - 45 43.87 2.6 430 n.m. n.m. - n.m. n.m. - n.m. - n.m. -

Note THS indicates top half-shell, BHS indicates bottom half-VKHOOįLQGLFDWHVrelative error, and n.m. indicates not measured.

84 Yvan Champoux and Stephan Odenwald / Procedia Engineering 72 ( 2014) 80 – 85

Fig. 2. Calibration measurement of the (a) top half-shell and (b) Fig. 3. Validation of the influence of the line of force application. bottom half-shell.

Table 3. Pilot test to evaluate the functionality of the new developed dynamometer.

measurement / task gripping force [N] supporting force [N]

#1 / no gripping, cycling with 50 W, vibration on 55 ± 3 54 ± 3 #2 / no gripping, cycling with 50 W, vibration off 58 ± 7 56 ± 7 #3 / gripping with 300 N, cycling with 50 W, vibration on 289 ± 38 171 ± 19 #4 / no gripping with 300 N, cycling with 50 W, vibration off 293 ± 25 175 ± 14 #5 / gripping at maximum strength, cycling with 50 W, vibration on 797 429 #6 / gripping at maximum strength, cycling with 50 W, vibration off 714 396 #7 / weight plate of approx. 10 kg, no cycling, vibration on 99 ± 0 99 ± 0.1 #8 / weight plate of approx. 10 kg, no cycling, vibration off 98 ± 6 98 ± 6.3

Note Vibration on indicates exposure to vibrations (frequency = 24.7 Hz, amplitude = 0.93 mm).

4. Discussion and Conclusion

The demands specified in the requirements list for the development of the novel dynamometer were mostly achieved. The dynamometer is capable of measuring gripping and supporting forces at the human-machine interface. The design allows usage on both sides of a handlebar, so that not only the forces of the right but also of the left hand can be measured. The relative error of the device over the whole measuring range is less than 5 %. Considering the measurement range as from 150 N to full scale the relative error is less than 2 %. Measurements performed with the novel dynamometer are reproducible with maximum deviations of ± 1 N. Nevertheless, the demand of monitoring forces independently from the line of force application could not be achieved. The dependency of the measurements from the line of force application might be resulting from a slight deviation in the positioning of the strain gages in relation to the neutral axis of the beams. Especially since the exact location and run of the neutral axis is unknown, because of the cross section skipping along the longitudinal axis of the half-shells from a cylindrical shape to a rectangular shape. Consequently, if a pure bending of one of the two half-shells is taking place, the strain gages will already be slightly twisted and producing a false strain signal. Another reason for the dependency could be that the strain gages among themselves were unsymmetrical mounted on the beams, which will also lead to a false strain signal. Nevertheless, if one considers the usual case of Yvan Champoux and Stephan Odenwald / Procedia Engineering 72 ( 2014) 80 – 85 85 application, i.e. measuring forces at a bicycle handlebar, the dependency from the line of force application will not preponderate. For instance, the smallest width of a female hand is 70 mm (DIN 33402-2). That means that the load application position is maximal 25 mm away from the centre of the dynamometer. Measurements with an approximately 10 kg weight plate indicated that the force deviation at an interval of ± 25 mm from the centre of the dynamometer was 6.9 N at maximum. The functionality of the new developed dynamometer was proven in a pilot test. The test demonstrated that the design is robust and that the device operates also while being exposed to vibrations. Hence, the novel dynamometer can be used in prospective studies to evaluate one of the two components of the athletes’ mechanical impedance during sporting activities, i.e. transmitted forces. In conclusion, a novel dynamometer was developed that overcomes the limitations of existing dynamometers with respect to the intended use at a bicycle handlebar.

Acknowledgements

The authors acknowledge Norbert Fahrack for his outstanding assistance within the construction process of the novel dynamometer. We are also very grateful to Steffen Müller for his assistance in the manufacture of the component parts and to Jacob Müller for editing the figure of the novel dynamometer.

References

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